BACKGROUND: Selective motor control (SMC), the ability to isolate selected muscle activation during a functional task, is often impaired. Gillette’s SMC scale is commonly used to classify the impairment level; however it may not be sensitive to muscle coactivation.AIM: To characterize differences in muscle activation levels and coactivation incidence in Gillette’s SMC grade levels.DESIGN: Non-randomized observational study.SETTING: Participants were recruited and examined in the motion analysis laboratory of a university hospital.POPULATION: Forty-two participants were enrolled: 23 patients with CP (13 females, 10 males; 15±5.59 years, range: 7-28 years; bilateral involvement; GMFCS levels I to III) and 19 able-bodied volunteers (14 females, 5 males; 22±1.54 years, range: 20-24 years).METHODS: Participants flexed each knee three times at self-paced velocity. Each limb was classified into one of three types using Gillette’s SMC scale: Type 0 (CP limbs with no ability to isolate movement), Type 2 (CP limbs with complete isolation of movement) and Type C (Control limbs of able-bodied volunteers). Surface electromyography recorded muscle activation levels of hamstring, rectus femoris, hip adductor, gastrocnemius and tibialis anterior muscles. We applied the Friedman ANOVA χ2 Test to analyze muscle co-activation incidence and Kruskal-Wallis ANOVA and Median Tests to analyze muscle activation levels. We used the Wilcoxon Matched-Pairs Test to compare results between SMC Types.RESULTS: Comparing mean activation levels of the majority of muscles, we found: CP limbs (Type 0+2) > Control limbs (P<0.001); Type 0 > Type 2 (P<0.05); and Type 2 > Type C (P<0.01). The incidence of muscle co-activation was affected by CP (P=0.008) and differed by SMC type (P<0.001).CONCLUSIONS: Our quantitative study confirmed that SMC is worse in Gillette’s Type 0 limbs than in Type 2 limbs. We also found that the SMC of Type 2 limbs of CP patients in CP patents is not equivalent to that of Type 2 limbs in able-bodied volunteers.CLINICAL REHABILITATION IMPACT: A better characterization of this clinical test will help gauge its usefulness in evaluating the effectiveness of rehabilitation treatments.